Electroluminescent devices based on organic semiconductors have gained a great deal of attention because of their high luminance

low drive voltage

and variety of emission colors.Different applications have different demands on the emitted light;sometimes colors are needed and in other cases it is necessary to have a bright white light source

especially for backlight applications in liquid crystal displays.Organic white light emitting diodes(LEDs)based on electroluminescent organic molecules have been reported using the microcavity technique

multilayer structures

multiple-quantum wells structures

or polymer LED based on polymer blends.However

most of the methods have the drawback that the chromaticity of emission coulor changes largely with the operating voltage

or the fabrication processes are more complex.We demonstrate efficient organic white light-emitting devices(LEDs)

using N

N-diphenyl-N

N'-bis(1-naphthyl)-(1

1-biphenyl)-4

4'-diamine(NPB)as hole-transporting layer

1

6-bis(2-hydroxyphenyl)pyridine boron complex((dppy)BF)as emitting layer

tris-(8-hydroxyquinoline)aluminum(Alq)as electron-transporting and chromaticity-tuning layer.This type of device has a simpler structure than those mentioned above

thus the fabrication process is much simpler.The white light comes from exciplex emission at the solid-state interface between(dppy)BF and NPB and from the exciton emission of NPB and(dppy)BF layers respectively.The chromaticity of white emission can be tuned by adjusting the thickness of Alq layer.The white LEDs with the Alq thickness of 15nm exhibit a maximum luminescence of 2 000cd/m

2

and efficiency of 0.58 lm/W

and the Commission Internationale De L'Eclairage(CIE)coordinates of resulting emission vary from(x=0.29

y=0.33)to(x=0.31

y=0.35)with increasing forward bias from 10V to 25V.The region is very close to equienergy white point(x=0.33

y=0.33).